[tor-talk] Reason Firefox version in TBB is so far behind?

[Joe Btfsplk]

Are there specific reasons for not using latest (or late-er) Firefox 
versions in Tor Browser Bundle?  Is it primarily because the latest 
version doesn't always work w/ Tor & fixes must be developed for Tor to 
deal w/ that?

I can understand that, but many of the changes in FF versions are 
security patches.  How does not using the FF version w/ latest security 
patches affect Tor users' security?